There are conventionally known techniques, for example, in which water is decomposed for obtaining hydrogen and oxygen by irradiation of a semiconductor material that functions as a photocatalyst with light (see Patent Literature 1, for example), or in which a surface of a base material is rendered hydrophilic by covering the surface of the base material with the semiconductor material (see Patent Literature 2, for example).
Patent Literature 1 discloses a method in which an n-type semiconductor electrode and a counter electrode are disposed in an electrolyte and the surface of the n-type semiconductor electrode is irradiated with light, so that hydrogen and oxygen are obtained from the surfaces of both electrodes. Specifically, use of a TiO2 electrode, a ZnO electrode, a CdS electrode and the like as the n-type semiconductor electrode is described therein.
Furthermore, Patent Literature 2 discloses a hydrophilic member formed of a base material and a coating layer formed on the surface of the base material, in which the coating layer has a titanium oxide layer that contains titanium oxide particles, and an island-shaped portion that is disposed on the titanium oxide layer and that is made of a second photocatalytic material other than titanium oxide. Specifically, use, as the second photocatalytic material, of a material having potentials at the bottom of a conduction band and the top of a valence band on a more positive side relative to the standard hydrogen electrode potential (negative side relative to the vacuum level) than the titanium oxide is described therein.
Moreover, there is proposed a photocatalytic thin film that brings about a highly efficient photocatalytic performance under natural light, in which at least one of metal ions such as Nb, V and Cr is injected into a photocatalytic thin film produced on a substrate to cause a change in the band gap or the potential gradient in the thickness direction of the photocatalytic thin film, thereby forming a gradient film (see Patent Literature 3).
Further, there also is proposed a technique for producing hydrogen by immersing, in a solution containing hydrogen sulfide, a multilayer thin film photocatalyst in which a first compound semiconductor layer and a second compound semiconductor layer that has a band gap different from the first compound semiconductor layer are disposed sequentially on an electrically conductive base material, and then irradiating this multilayer thin film photocatalyst with light (see Patent Literature 4).